A contactless power transfer device uses electromagnetic induction between a primary coil (power transmission coil) and a secondary coil (power reception coil) to supply power from the power transmission coil to the power reception coil. The contactless power transfer device may be used to charge a secondary battery which is mounted in an electric vehicle or a plug-in hybrid vehicle and it is expected that a demand for a contactless power transfer device for charging a vehicle will be increased in the near future.
In the case of a contactless power transfer device for charging a vehicle, a vehicle with a power reception coil mounted on a bottom surface of a floorboard comes to a stop at a position where the power reception coil is disposed directly on a power transmission coil provided on the ground to perform the contactless power feeding. However, due to misalignment of the power transmission coil and the power reception coil in a horizontal direction or variation in a gap length in a vertical direction, a coupling factor between the power transmission coil and the power reception coil varies.
The following Patent Literature 1 suggests a contactless power transfer system in which a change in a power feeding efficiency of the contactless power transformer (power transmission coil+power reception coil) is made to be small even when the coupling factor varies.
As illustrated in FIG. 14, the system includes a full-wave rectifier 161 which converts a commercial alternating current 164 (AC) into a direct current, a full-bridge inverter 163 which generates a high frequency alternating current from the direct current, a contactless power transformer 1410 which performs contactless power feeding, a full-wave rectifier 1471 which converts a secondary-side alternating current output into a direct current, and a load 1472 which is a secondary battery.
In this system, the commercial alternating current 164 is converted into the direct current by the full-wave rectifier 161; the high frequency alternating current is generated from the direct current by the full-bridge inverter 163 to be fed into the contactless power transformer 1410; and the secondary-side alternating current output is converted into the direct current by the full-wave rectifier 1471 to supply the power to the load 1472 (secondary battery).